When I first started to strength train in high school, one of the first things I remember reading in the weight room was this giant chart that highlighted areas of so-called fast-twitch and slow-twitch muscle fibers in the body. Lots of "the boys," that is, the friends that I trained with, took this apparent knowledge to heart, and for different looks each of us wanted to achieve, we designed different workouts based on whether they would fatigue our "slow-twitch" or "fast-twitch" muscles. For example, those of us who wanted a lithe look of the runner did low-weight, high repetition workouts so that we might fatigue the "slow-twitch muscles", and those who wanted a more bulky, cut look of a sprinter performed more high-weight, short-repetition workouts to fatigue the "fast-twitch" muscles.* This information was particularly relevant for those few of my friends who wanted to try out for various positions on the football team, which as everyone knows weight on stamina, endurance, and power differently.

Unfortunately, the study of fitness and nutrition as a scientific discipline is replete with misinformation and pseudoscience, to the point that one can't really Google for even basic information without being afraid that one is being marketed to or upsold in some fashion. Still, I distinctly remember reading later somewhere that either "fast-twitch" and "slow-twitch" muscles were pseudoscience or, at the very least, outdated terms, much in the same way "alpha dog" is an outdated term for those who study dominance hierarchies among wolves, but a term which still persists in the common parlance. Alas, I can't remember exactly, and lots of different websites in my search results seem to take "fast-twitch" and "slow-twitch" seriously.

So here are my canonical questions:

Are "fast-twitch" and "slow-twitch" actual scientific terms? If they were so, are they so now?

If they are indeed scientific, does knowing about them help one design better workouts? E.g.,

Does one have a natural endowment or ratio of "fast-twitch" to "slow-twitch"? Can it be determined practically?

If not the above, is it possible to convert one type of muscle to another in some way?

Do certain muscles contain more of one muscle fiber than other muscles? Is that relevant to designing workouts?

Do members of certain athletic professions (sprinters, swimmers) possess different ratios of fast-twitch to slow-twitch than others?

(*) I am well aware that many strength trainers recommend high-weight, low repetition workouts regardless of which look one wants to achieve. That is not what I'm asking. I'm asking whether the rationale that we came up with was sound.

3 Answers
3

Yes, there are two different types of muscles and knowing the differences between them is important

Type I, slow oxidative, slow twitch, or "red" muscle is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red color. It can carry more oxygen and sustain aerobic activity.

Type II, fast twitch muscle, has three major kinds that are, in order of increasing contractile speed:[4]

Type IIa, which, like slow muscle, is aerobic, rich in mitochondria and capillaries and appears red.

Type IIx (also known as type IId), which is less dense in mitochondria and myoglobin. This is the fastest muscle type in humans. It can contract more quickly and with a greater amount of force than oxidative muscle, but can sustain only short, anaerobic bursts of activity before muscle contraction becomes painful (often incorrectly attributed to a build-up of lactic acid). N.B. in some books and articles this muscle in humans was, confusingly, called type IIB.[5]

Type IIb, which is anaerobic, glycolytic, "white" muscle that is even less dense in mitochondria and myoglobin. In small animals like rodents this is the major fast muscle type, explaining the pale color of their flesh.

Are "fast-twitch" and "slow-twitch" actual scientific terms? If they were so, are they so now?

Yes. Read the quote above.

If not the above, is it possible to convert one type of muscle to another in some way?

No, although targeting specific workouts will either increase the mass of one, the other, or both. It all depends on how you train.

Do certain muscles contain more of one muscle fiber than other muscles?

Most likely, but that depends on each and every muscle. You'd probably need to check on a muscle by muscle basis.

Is that relevant to designing workouts?

Not really. Just understanding that there are two different types and specific workouts will target either one or the other is enough.

Do members of certain athletic professions (sprinters, swimmers) possess different ratios of fast-twitch to slow-twitch than others?

Most definitely. And, it's important to know which types. Sprinters, will be targeting Type II (fast twitch) meaning that they want to focus more on a regime of High Intensity Interval training to increase their anaerobic threshold and build muscle mass for power. Swimmers are more concerned with endurance so they'll focus more on endurance workouts.

The great part about knowing which muscle fiber type your targeting you can supplement training with other types of workouts to maximize the benefits. For instance, if you play football you'll want to maximize power in shorter timer intervals so you can practice by doing hard high intensity sprints to increase your anaerobic threshold.

If you're a swimmer and you don't have access to a pool, you could to aerobic exercise by running long distances at a moderate pace to increase your endurance.

Note: Swimmers isn't exactly the best example of slow-twitch because they need a good combination of both for power and endurance.

There are benefits to both and whichever you choose depends completely on what your goals are.

There's even a specific gene, ACTN3, that's linked with your muscle fiber twitch type. If you have the non-mutated form of the gene, you'll have an abundance of slow-twitch muscle fibers. If you have the mutated form of the gene, you'll have an abundance of fast-twitch muscle fibers.
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user2567Dec 22 '11 at 19:32

You need to have your muscle type determined. You can do it by genetic testing.
You can have a CC, CT, or TT genotype on marker rs1815739 (Humans have two instances of each chromosome).

CC Two working copies of alpha-actinin-3 in fast-twitch muscle fiber. Many world-class sprinters and some endurance athletes have this genotype.

CT One working copy of alpha-actinin-3 in fast-twitch muscle fiber. Many world-class sprinters and some endurance athletes have this genotype.

TT No working copies of alpha-actinin-3 in fast-twitch muscle fiber. Few world-class sprinters have this genotype, but many world-class endurance athletes do.

After you got your result, it might not be that helpful.

Lastly, muscle fiber only contributes a small part to your overall
athletic performance. Other physical characteristics, such as lung
capacity, and behaviors, such as regular exercise, also make important
contributions to your prowess in sports.

[...]

One study looked at a group of Greek teenagers who had been tested for
a variety of fitness measures related to power and endurance sports.
In this group, ACTN3 genotype had no effect on the girls, but boys
with the TT genotype were significantly slower in a 40 m sprint.
Interestingly, running was the only power event that the different
versions of ACTN3 seemed to affect. For activities like throwing a
basketball or jumping into the air, performance was unaffected by
genotype.

Running involves repeated use of the muscles, while jumping only uses
muscles once: damage is not an issue.
[...]
Alpha-actinin-3 may also affect athletic performance by virtue of its
effects on oxygen usage in muscle.

These are quotes from a web page on the ACTN3 gene, from a genome typisation company. (This content is behind a paywall, so I cannot link to it)

Muscle is muscle. However there are multiple pathways for muscle to expend energy and these fit into your question, I think.

Phosphagen: typical activities that
use this system to metabolize ATP are
explosive, require maximal effort and
are very short in duration. A barbell
snatch is a good example.

Glycolytic: a typical activity is
wrestling, which requires a sustained,
intense effort, but not maximal. Use
of this pathway causes lactic acid
build up, which must be addressed in
training.

Oxidative: if enough oxygen is
available, the glycolytic pathway is
diverted to the oxidative system. The
most familiar activities that use this
system are low intensity, long
distance activities like marathon
running and XC skiing. ATP recovery is
very high in the oxidative system.